Acoustic material

ABSTRACT

This invention provides an acoustic material having high elastic modulus and large internal loss by subjecting a high-modulus stretched polyethylene containing paraffin wax to plasma treatment. When the acoustic material of the present invention is used for a diaphragm of a speaker, for example, it is possible to suppress the fluctuation of frequency characteristics resulting from split vibration, decrease harmonic distortion and improve transient characteristics.

TECHNICAL FIELD

This invention relates to an acoustic material employed as the diaphragmfor a loudspeaker and more particularly to an arrangement for improvinginternal losses in the acoustic material consisting essentially of thedrawn polyethylene having a high modulus of elasticity.

BACKGROUND OF ART

The acoustic material employed in the diaphragm of a loudspeaker isrequired to have low density, high modulus of elasticity and hence ahigh rate of propagation of longitudinal waves and large internallosses, for enhancing the reproduction frequency range. With this inview, evolution towards industrial application of a so-called compositediaphragm is now underway using a variety of fibers such as carbon-,aramide-, glass- or polyolefin resin fibers as the reinforcingmaterials.

Above all, drawn high elastic modulus polyethylene, prepared by acrystal surface growth method, gel spinning-ultradrawing method or amelt draw orientation method is thought to be suitable as the acousticmaterial, in that it has a lower density and a higher rate ofpropagation of longitudinal waves. For example, it is shown in theJapanese Patent Publication KOKAI No - 182994/1983 to use polyethylenefibers having the rate of propagation of the longitudinal waves notlower than 4000 m/sec as the acoustic material

It is noted that the aforementioned high elastic modulus polyethylenefibers compare favorably with aluminum in elastic modulus (Young'smodulus), but are inferior to polyester in internal losses (tan δ), asshown in Table 1 indicating the physical properties thereof, such thatit cannot be used directly as the acoustic material, above all, as theloudspeaker diaphragm.

                  TABLE 1                                                         ______________________________________                                                        Young's                                                                 tanδ                                                                          modulus  method of preparation                                ______________________________________                                        polyethylene                                                                            a     0.013   47     fibrilated crystal                             fibers    b     0.011   82     growth, gel spinning-                                    c     0.014   78     ultra drawing, or                                                             melt spinning orienta-                                                        tion                                           aluminum        0.008   73       --                                           polyester       0.053    5     biaxially drawn film                           ______________________________________                                    

The present invention has been made in view of the above describeddeficiencies of the prior art and is aimed to provide an acousticmaterial which is improved in internal losses without impairing the highmodulus of elasticity proper to the drawn high elastic moduluspolyethylene and which is relatively free from higher harmonicdistortion or from fluctuations in the frequency response, that is,crests and valleys, caused by split vibrations, when the acousticmaterial is used as the diaphragm material.

DISCLOSURE OF THE INVENTION

As a result of our eager and perseverant investigations towardsimproving the internal losses of the drawn high elastic moduluspolyethylene, the present inventors have found that it is most effectiveto process drawn high elastic modulus polyethylene containing paraffinwax as the damping agent with plasma.

On the basis of this finding, the present invention provides an acousticmaterial which is characterized in that drawn high elastic moduluspolyethylene containing 1 to 5 wt. % of paraffin wax obtained by, forexample, melt draw orientation, is processed with plasma, and in that atleast a portion of paraffin wax contained in said drawn high elasticmodulas polyethylene is not extracted with boiling n-hexane.

The drawn polyethylene, a main constituent of the acoustic material ofthe present invention, is prepared by medium to low pressurepolymerization of ethylene either singly or with a minor quantity ofother α-olefins, such as propylene, 1-butene, 4-methyl-1-pentene or1-hexene. It has higher modulus of elasticity, such as the initialtensile elastic modulus not less than 30 GPa and preferably not lessthan 50 GPa and fracture elongation not higher than 6% and preferablynot higher than 4%, thanks to the high degree of orientation of thepolyethylene molecular chain brought about by ultra drawing. Above allthe drawn polyethylene prepared from ultra high molecular weightpolyethylene having an intrinsic viscosity (η) in a decalin solvent at135° C. of not lower than 5 dl/g and preferably 7 to 30 dl/g, isobviously preferred since it is superior in tensile elastic modulusretention and in tensile strength retention at higher temperatures.

Since the drawn polyethylene as mentioned hereinabove is required tocontain paraffin wax therein, it is preferably prepared by the so-calledmelt draw orientation method. This method is described for example inthe Japanese Patent Publication KOKAI No. 187614/84 and includes thesteps of melting and kneading a mixture of the aforementioned highmolecular weight polyethylene and paraffin wax by a screw extruder at atemperature of 190° to 280° C., extruding the undrawn material from adie maintained at 210° to 300° C., drafting the material at a draftratio at least above unity, cooling and solidifying the material anddrawing the cooled and solidified material at a temperature of 60° to140° at a draw ratio not less than three.

The paraffin wax employed mainly contains saturated aliphatichydrocarbons having preferably the molecular weight of not higher than2000 and the melting point of the order of 40° to 120° C. Morespecifically, the paraffin wax may include n-alkanes having 22 or morecarbon atoms, such as docosane, tricosane, tetracosane or triacontane, amixture containing these n-alkanes as main component and lowern-alkanes, paraffin wax separated and refined from petroleum, low tomedium pressure polymerized polyethylene wax, high pressure polymerizedpolyethylene wax, or ethylene copolymer wax which is a low molecularweight polymer of ethylene, either singly or as a copolymer with otherα-olefins, low molecular weight wax obtained from polyethylene such asmedium to low pressure polymerized polyethylene and high pressurepolymerized polyethylene by thermal degradation, oxides of these waxesand modified products of these waxes by maleic acid.

At least a portion of the aforementioned paraffin wax is contained inthe aforementioned drawn polyethylene and plays the role of a dampingagent by physico-chemical processing, viz. the plasma processing.

The method of plasma processing consists in effecting glow discharge inplasma gas in the presence of an organic compound, herein a paraffinwax, to produce an excited compound and either having the excitedcompound contained in the drawn polyethylene after the modification ofthe compound or polymerizing the excited compound with the drawnpolyethylene. In the plasma processing, the impressed voltage and thegas pressure may be preset in the usual ranges and it does not matterwhat kind of the plasma is to be employed.

This plasma processing will result in improved surface properties,adhesiveness in particular, of the drawn polyethylene, and is mostadvantageous when, for example, the polyethylene is conjugated withother materials to produce an acoustic material.

It is preferred that the amount of the paraffin wax remaining in thedrawn polyethylene after the plasma processing be in the range from 1 to5 wt. %. With the amount of the residual paraffin wax less than 1 wt. %,the damping effect is insufficient. With the amount in excess of 5 wt.%, the Young's modulus is undesirably lowered.

The paraffin wax is dissolved in the drawn polyethylene prepared by, forexample, the melt draw orientation method. When the drawn polyethyleneis subjected to plasma processing, the wax plays the role of the dampingagent to increase the internal losses.

At this time, the drawn polyethylene itself is not lowered in thephysical properties but the higher rate of propagation of thelongitudinal waves is maintained with the high modulus of elasticity andlow density.

It should be noticed that not all of the paraffin wax remaining in thedrawn product is modified or polymerized with the drawn polyethylene. Itis inferred that modification or polymerization occurs only in theregion of 10 to 30 Å from the surface of the drawn polyethylene, withthe wax deep within the drawn product remaining intact withoutundergoing any reaction. It is noted that the surface of the drawnpolyethylene in which the paraffin wax is modified and caused to remainor polymerized has a densely packed structure, so that there is noopportunity for the wax remaining deep in the drawn product to bedeposited on the surface of the product.

Therefore, when the acoustic material of the present invention is usedin, for example, a diaphragm for a loudspeaker, it becomes possible tosuppress fluctuations in the frequency response brought about by splitvibrations, while reducing the distortion due to secondary harmonics andimproving transient characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a characteristic diagram indicating the difference in thereproduction frequency response of the diaphragm caused by the presenceor absence of the plasma processing treatment of the high elasticmodulus polyethylene fibers containing paraffin wax. FIG. 2 is acharacteristic diagram showing the difference in the frequency responseof the distortion by second order harmonics.

PREFERRED EMBODIMENT TO PRACTICE THE INVENTION

The present invention will be explained on the basis of concrete testresults

Preparation of Polyethylene Fibers

A 25:75 blend of an ultra high molecular weight polyethylene having aintrinsic viscosity η in the decalin solvent at 135° C. equal to 8.20dl/g and a paraffin wax having a melting point of 60° C. and a molecularweight of 460 was melt-spun and drawn under the following conditions.

Thus the powders of the ultra high molecular weight polyethylene andpulverized paraffin wax were mixed, melted and kneaded together at aresin temperature of 190° C. using a screw extruder 20 mm in diameterand a L/D ratio equals to 20. The melted product was then extrudedthrough a die having an orifice diameter of 1 mm and solidified withcold water of 20° C. at an air gap of 10 cm. The drafting was performedat this time so that the diameter of the cooled and solidified fiber orfilament be 0.50 mm, that is, with a draft ratio equal to two. The termdrafting herein means the drawing of the melted product while it isextruded from the screw extruder in the molten state, while the termdraft ratio means the ratio of the die orifice diameter to the diameterof the cooled and solidified fiber or filament.

Then, using a pair of godet rolls, drafting was continuously performedin a drafting vessel containing n-decane as the heat medium, with thetemperature in the vessel equal to 130° C. and the vessel length equalto 40 cm.

The drawn product was then processed with n-hexane and the amount of theremaining paraffin wax was controlled.

Ascertainment of Immobilization of Paraffin Wax by Plasma Processing

In accordance with the above process, polyethylene fibers (samples 1 and2) containing 6 wt. % and 2.5 wt. % of paraffin wax, respectively, wereprepared and immobilization of a portion of a paraffin wax caused byplasma processing was ascertained from the amounts of extraction byn-hexane before and after the plasma processing.

The plasma processing was performed under conditions of an argon plasmagas pressure of 0.04 Torr, 100 mA and 240 V.

Paraffin wax was extracted with n-hexane for 24 hours using a Soxhlet'sextractor.

The residual amounts of paraffin wax remaining before and after plasmaprocessing are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        amount of extrac-                                                                              amount of extrac-                                                                           re-                                            tion before plasma                                                                             tion after plasma                                                                           sidual wax                                     processing (wt. %)                                                                             processing (wt. %)                                                                          in filament                                    ______________________________________                                        sample 1                                                                             6.0           2.6           3.4                                        sample 2                                                                             2.5           1.2           1.3                                        ______________________________________                                    

It is seen from the Table 2 that the wax not extracted with n-hexaneafter plasma polymerization remains in the filament in an amount ofabout 50%. Thus it has been demonstrated that a portion of the wax hasbecome immobilized on the polyethylene fibers by the plasma processing.

Ascertainment of the Damping Effect

Using polyethylene fibers previously subjected to plasma processing(samples 1 and 2) and polyethylene fibers (reference sample) notsubjected to plasma processing, unidirectional conjugation was performedwith an epoxy resin, and the physical properties of the conjugate orcomposite material were measured and compared by the vibration reedmethod. The following conjugating conditions were adopted.

Conjugating conditions

Polyethylene fibers: 1000 deniers; 200 filaments

epoxy resin: YD 128 by Toto Kasei KK

hardener: 2E4MZ by Shikoku Kasei KK

The results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                               paraffin                 vol. percent.                                        wax             Young's  of fibers in                                         content         modulus  the conjug.                                          (wt. %)                                                                              tanδ                                                                             (GPa)    mat.                                          ______________________________________                                        Sample 1 3.4      0.038    50.3   0.63                                        Sample 2 1.3      0.026    73.2   0.65                                        reference                                                                              0        0.017    70.4   0.63                                        sample                                                                        ______________________________________                                    

It is confirmed from this Table that the composite fiber material towhich the present invention is applied (samples 1 and 2) has largerinternal losses (tan δ) such that it is sufficiently suited as theacoustic material, especially the diaphragm material. It is noted that,since the present invention is aimed to provide the acoustic materialthe effects of the fibers were checked by evaluating the compositematerial instead of evaluating the polyethylene fibers or filaments perse.

Evaluation as the Diaphragm

Using polyethylene fibers previously processed with plasma (sample 2)and polyethylene fibers not processed with plasma (reference sample), adiaphragm for a full range speaker unit, 16 cm in diameter, was preparedunder the following conjugating conditions, and the reproductionfrequency response as well as the frequency response for the secondharmonic distortion was measured.

Conjugating Conditions

polyethylene fibers: 1000 deniers; 200 filaments (used as the flat wovenfabric of 150 g/m²)

epoxy resin: YD 128, by Toto Kasei KK

hardener: 2E4MZ, by Shikoku Kasei KK

The results are shown in FIGS. 1 and 2. In these figures, line iindicates the characteristics of the diaphragm prepared with thepolyethylene fibers subjected to plasma polymerization and line iiindicates those of the diaphragm prepared with the polyethylene fibersnot subjected to plasma polymerization.

As a result, it has been shown that the diaphragm prepared with thepolyethylene fibers subjected to plasma processing exhibits a peak inthe high limit reproduction frequency which is lower than that of thediaphragm prepared with the polyethylene fibers not subjected to plasmaprocessing, while undergoing lesser distortion due to secondaryharmonics in the overall range so that there are obtainedcharacteristics reflecting the effects of the acoustic material of thepresent invention.

We claim:
 1. An acoustic material consisting essentially of drawn highelastic modulus, polyethylene including a paraffin wax and beingsubjected to an electrical plasma surface treatment.
 2. An acousticmaterial consisting essentially of drawn high elastic moduluspolyethylene containing 1 to 5 wt. % of paraffin wax and saidpolyethylene having the surface thereof being processed with anelectrical plasma surface treatment.
 3. An acoustic material accordingto claim 2, wherein at least a portion of the paraffin wax is caused toremain in the drawn high elastic modulus polyethylene after extractionwith boiling n-hexane.
 4. An acoustic material according to claims 2 or3, wherein the paraffin wax is at least one of n-alkane, paraffin wax,polyethylene wax, oxidized wax, and maleic acid modified wax.
 5. Anacoustic material according to any one of claims 1 to 3, wherein thedrawn elastic modulus polyethylene has the initial tensile elasticmodulus of not less than 30 GPa and the fracture elongation of nothigher than 6%.
 6. An acoustic material according to any one of claims 1to 3, wherein the drawn high elastic modulus polyethylene is a drawnproduct of high molecular weight polyethylene having a intrinsicviscosity in a decalin solution at 135° C. of not less than 5 dl/g. 7.An acoustic material according to any one of claims 1 to 3, wherein thedrawn high elastic modulus polyethylene is prepared by a melt draworientation process.
 8. An acoustic material comprising drawn highelastic modulus polyethylene including paraffin wax, at least a portionof the paraffin wax remains after extraction with hexane, the materialbeing subjected to an electrical plasma surface treatment.
 9. Anacoustic material according to claims 2 or 3, wherein the paraffin waxcontains saturated aliphatic hydrocarbons having a molecular weight thatis not greater than 2000 and a melting point ranging from 40° C. to 120°C.